Enable addition of module using basis_add_subdirectory

[schuhschuh]

Instead of basis_add_module, it should be possible to use basis_add_subdirectory to add a directory that contains a BASIS project module, i.e., a BasisProject.cmake file. In this case, PROJECT_IS_MODULE must be set to TRUE before the add_subdirectory and to FALSE again afterwards. This is useful when specifying a module directory as one of the directories of the project rather than having it as part of the list of modules that can be enabled/disabled (auto-addition of MODULE_<Name> option).

[schuhschuh]

Implemented by 15d5c4a8ef37a08a5416092b468fd951e61f97c6.